Gibberellins (GAs) are a class of plant hormones that affect several important processes such as seed germination, stem elongation, and flowering. Over 100 GAs have been identified in plants and fungi. GAs are synthesized through the mevalonic acid pathway and non-mevalonic acid pathway. Their biosynthesis involves multiple oxidation and hydroxylation steps. GA signaling involves perception by receptors, signal transduction through second messengers, and regulation of gene expression by transcription factors such as GAMyb. Mutations in GA biosynthesis, signaling, and response genes have helped elucidate the complex GA pathways and gene networks.
4. a class of plant hormones
affect several important plant processes
eg., seed germination
stem elongation
flowering
male sterility
Gibberellins (GAs)
8. Gibberellic acid (GA3)
End metabolic product in fungi
Plant GA20 GA5 GA3
Commercial
High activity
Slow degradation
Similar to GA1
additional double bond
9. Gibberellins
GA4 GA7 nonpolar, slowly diffuse
GA9 GA12 precursor
GA29 GA34 deactivated form
Different tissues
Different forms of GA
16. In higher plants
from GA12 aldehyde
Early 13-hydroxylation pathway
(GA1)
Non 13-hydroxylation pathway
(GA4)
with GA20oxidase genes:
pathway shifted
GA4 increased / GA1 decreased
18. Vegetative tissue: conserved synthetic pathway
13-OH pathway to GA20 (C19-GA)
then 3β-OH to GA1
except: arabidopsis and cucumber
non 13-OH pathway to GA4
Reproductive tissue/seed: various pathways
different forms of GA
19.
20. From mevalonic acid (6C)
GGPP (20C-linear cpd)
ent kaurene (1st specific cpd)
GA12 aldehyde (first GA)
GAx
21.
22.
23. Isoprene (5C) as basic unit
ent-Gibberellane skeleton
tetracyclic diterpenoid cpd
Gibberellins
24. 2 main types:
C20-GA and C19-GA
GA derivatives by modification of 4 rings
* C20 oxidation: CH3 CH2OH CHO COOH
* Hydroxylation at C2 C3 and C13:
number, position
stoichiometry
* Loss of C20 (C20 to C19 GA)
25.
26. * 2β-OH: GA20 GA29
GA1 GA8
* C20 oxidation to COOH
GA inactivation
27. * Conjugation by glucose
Glycosylation:
inactive, storage and transport
Glucose via COOH: GA glycoside
Glucose via OH: GA glycosyl ether
GA inactivation
28. GA synthesis mutants
Pea na mutant: dwarf
ent-kaurene GA12 aldehyde
Pea le mutant: dwarf
exogenous GA1 tall
exogenous GA20 no response
cloned Le gene:3β hydroxylase
GA20 GA1
29. Considering 2 loci
na Le normal ent-kaurene
Na le normal GA20
Grafting
1. na Le scion
Na le stock tall
2. Na le scion
Na Le stock dwarf
Conclusion?
31. By (may)
decrease Ca concentration in the wall
increase Ca uptake into the cell
reduce crosslinking of lignin-related cpd
(via peroxidase)
GA mechanism in elongation
32. GA mechanism in germination
Activate transcription of
α amylase gene
In scutellum and aleurone
33. GA detection and assay
Bioassay
Easy but not specific
Fractionation
Plant response
Lettuce hypocotyls elongation
Microdrop/dwarf rice
α amylase production
37. At last step
Gene expression
Specific region in promoter
cis element
DNA-binding protein
transcription factor
38. Exogenous GA / GA inhibitor
GA mutant
Gene identification / Gene cloning
Gene expression / Transformation
GA studies
39. Enzyme: gene product of multigene family
Each gene with specific pattern of expression
AtGA20ox1: shoot growth
AtGA20ox2: inflorescence development
AtGA20ox3: early seedling development
GA synthesis
40. Genes controlled by GA, light and daylength
GA: inhibit transcription of GA20oxidase
(GA19 to GA20)
inhibit 3β hydroxylase
promote 2β hydroxylase
At later steps of synthetic pathway
41. Light: promote conversion of GA1 to inactive GA8
reducing shoot elongation
Negative feedback: reduce production of active GA20 and GA1Daylength (LD): floral initiation
activates GA20oxidase activity
GA53 to GA44
GA19 to GA20
42. Lettuce: Lactuca sativa seed germination
Red light: activates LsGA3ox1 expression
GA1 increase
Far-red light: inhibits LsGA3ox1
Auxin: promote GA1 production
inhibit deactivation steps to GA29 and GA8
Pea, Pisum sativum
In de-etiolated pea seedling, exposed to
red, blue, far red, all reduce GA1 level
43. Arabidopsis:
seed germination assay
5 complementation groups (56 lines)
ga1 ga2 ga3 ga4 and ga5
all recessive, dwarf, and male sterile
ga1 and ga2 reversed by ent-kaurene
ga3 reversed by ent-kaurenal
GA synthetic mutants
53. 8 dominant alleles with different severity
Dwarf: prevent lodging
Wheat + N fertilizer: increase yield
increase height
Norin10: dwarf line
2 mutated loci: Rht1 or Rht-B1b (chrs 4B)
Rht2 or Rht-D1b (chrs 4D)
Wheat Rht mutant
54. All genes cloned:
deduced amino acid sequence
GAI / Rht / d8 homologs
Conserved domains I and II in N terminal
gai mutant: deletion in domain I
D8 / Rht: mutation in domain I and/or II
*N terminal essential for GA response*
55. Similar to WT + GA
Tall by elongated internodes
Arabidopsis spy rga
Barley sln spy
Rice slr
Tomato pro
Pea la crys
Recessive / Negative regulators
Increased signal transduction mutants
56. Arabidopsis rga
Identified by suppression analysis of ga1-3
New mutant: taller
ga1-3 < ga1-3* < WT
new locus: repressor of ga1-3 (rga)
Increased signal transduction mutants
57. rga: recessive (deletion mutation)
increase stem elongation
reverse ga1-3 delayed flowering time
no effect on GA biosynthesis
RGA: negative regulator
Gene: 82% homology to GAI
especially in N region
Increased signal transduction mutants
58. Original gai mutant: gain of function
Loss of function allele of GAI ?
Phenotype: normal
Increase paclobutrazol resistance
Low GA = normal height
59. At least two components in
Arabidopsis GA signaling pathway
GAI and RGA
homopolymeric Serine / Threonine residue
leucine heptad for protein-protein interaction
putative nuclear localizing signal
60. slender mutant
recessive
long internodes and narrow leaves
male sterile
increase α-amylase w/o GA
low endogenous GA
resistant to GA synthesis inhibitors
Barley sln
61. negative regulator
sln x dwarf mutant = sln phenotype
SLN = GAI/RGA homolog
Dominant allele of SLN mutant
Mutation in N terminal
Dwarf barley
68. Barley: HvGAMyb
Bind specific sequence in
promoter of α-amylase gene
Increase gene expression
Overexpression of HvGAMyb gene
= GA treatment
Transcription factor: GAMyb
69. Arabidopsis: GAMyb-like genes
AtMyb33 AtMyb65 AtMyb101
Functional homologs of barley GAMyb
Transform barley aleurone with AtMyb33
Activate α-amylase production
70. Arabidopsis: facultative LD plants
Transfer plants from SD to LD
11x increase of GA1
3x increase of GA4
increase AtMyb33 expression
in shoot apex
shoot apex transition to flowering
71. Potential target for AtMyb
LFY promoter
LEAFY: meristem-identity gene
Evidence AtMyb binding
to a specific 8-bp sequence
in LFY promoter
72. cis elements
specific regions in promoter
transcription factor binding site
identified by deletion or
site specific mutagenesis:
gene expression after promoter modification
74. GA and α-amylase production
Perception at membrane receptors
Increase intracellular Ca
Decrease intracellular pH
Increase [CaM]
Increase cGMP
Increase GAMyb transcription
Increase α-amylase activity
Some protein phosphorylation